Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM) is a high-speed switching technology whose roots trace back to broadband ISDN initiatives conceived in the 1980s by organizations like the ITU-T. Originally designed with the high-speed transfer of voice, video, and data over public networks in mind, the scope of ATM has broadened to include transmission over private LANs and WANs. Typical transmission rates on ATM networks range from 155Mbps up to multi-gigabit speeds. ATM is capable of running over a variety of network media including fiber optics and UTP.

Although often referred to as a packet switching technology, ATM is best described as a hybrid that includes elements of both circuit and cell switching. While the interconnections between ATM switching equipment can be used to provide multiple redundant paths over which data can travel, ATM uses virtual circuits to establish dedicated connections between network endpoints.

The main protocol data unit of ATM is technically not a packet. ATM instead uses fixed-length “cells” as its method of encapsulating data to be transferred over ATM networks. ATM cells are relatively small at only 53 bytes in length. They include a 5-byte header and 48-byte payload, as illustrated in the figure below. Their small and fixed size makes them perfect for the transmission of time-sensitive traffic like voice and video. This is a superior alternative to traditional packets, whose variable lengths introduce added latency to network communication processes.

An ATM cell is always 53 bytes in length, comprised of a 5-byte header and 48-byte payload.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.